Back-up warning signal



FIG. 1 is a perspective view of a back-up warning signal showing my new design;

FIG. 2 is a front view of the invention;

FIG. 3 is a rear view of the invention;

FIG. 4 is a left view of the invention;

FIG. 5 is a top view of the invention; and,

FIG. 6 is a bottom view of the invention. 

The ornamental design for back-up warning signal, as shown and described. 